1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus including the same, and more specifically, to a sheet feeding apparatus that separates sheets one by one and feeds the sheets and an image forming apparatus including the same.
2. Description of the Related Art
Conventionally, an image forming apparatus includes a sheet feeding apparatus that separates sheets stacked on a sheet stacking plate one by one from the top of the stack and feeds the sheets to the image forming apparatus. The sheet feeding apparatus urges the sheet stacking plate, which is provided to be movable up and down, against a feeding roller by a coil spring to thereby contact the top surface of the stacked sheet with a feeding follower roller rotatably supported about the same axis as the feeding roller. Subsequently, the feeding roller rotates, thereby contacting the friction portion provided on the feeding roller with the sheet, which is fed while being separated one by one by a separation pad provided in a manner pressed against the friction portion. This technique is disclosed in Japanese Patent Application Laid-Open No. H07-257765.
In recent years, demands for a thin sheet of paper (hereinafter referred to as a “thin sheet”) containing small amount of pulp (low basic weight) has increased because of increase in awareness of the environment. However, the thin sheet has a low rigidity. Accordingly, there is a problem of easily causing a jam because of difficulty in feeding in a case of using the sheet in a conventional, compact, low-cost image forming apparatus.
Here, a jam of a thin sheet is described with reference to FIGS. 16A to 16C. As illustrated in FIG. 16A, in a conventional image forming apparatus, even if the sheet is subjected to double feeding according to a dragging phenomenon due to a frictional force between sheets in an immediately previous feeding step, the sheet is separated at a separation pad 19. Accordingly, a sheet S1 sometimes remains between the separation pad 19 and the feeding follower roller 18.
In this state, when a feeding roller 15 rotates in the direction of an arrow R1 and a sheet supporting portion 3 rotates in the direction of an arrow R2 by an urging force of a feeding spring 5 according to a feeding signal from a control part, the sheet S1 is sandwiched between a sheet stack T and the feeding follower roller 18 as illustrated in FIG. 16B. When the feeding roller 15 rotates from the state illustrated in FIG. 16B in the direction of arrow R1, the tip of a friction portion 15a of the feeding roller 15 abuts against the sheet S1 as illustrated in FIG. 16C.
At this time, as illustrated in FIG. 16D, the sheet S1 is subjected to a frictional force in the direction of an arrow F1 at an abutting position P1 where the sheet abuts on the tip of the friction portion 15a of the feeding roller 15, and the tip is subjected to a frictional force in the direction of an arrow F2 from the separation pad 19. Accordingly, if the rigidity of the sheet S1 is low, the frictional forces cause buckling of the sheet S1. With buckling of the sheet S1, the image forming apparatus easily causes a jam.
In particular, a compact and low-cost image forming apparatus (sheet feeding apparatus) easily causes the dragging phenomenon. Accordingly, with a thin sheet for which the demand is assumed to increase, there is a possibility that a jam due to buckling causes a problem more than the present state. On the other hand, it can be considered that the problem of a jam due to buckling is solved by providing a sheet separation device for preventing the dragging phenomenon of a sheet. However, the device requires a complicated mechanism. Accordingly, it is difficult to realize the device with an inexpensive configuration, and it is difficult to mounting the device on a compact and low-cost image forming apparatus.